There is a sensor that can tell you accurately what your mud and...

There is a sensor that can tell you accurately what your mud and formation pressure is....it's called a formation test tool.

Some of the basic LWD tools include a pressure sensor that gives real time readouts of down hole mud density (ECD) and an LWD formation test tool would allow you to take formation pressures at stations as you drill. The problem is that by the time you have the FPWD tool probe into the reservoir you will have drilled way past the top of same (the sensor is quite a fewmetres behind the bit) and taken a kick if your mud system is way underbalanced.